The present invention relates to improved thermal relays, particularly for use in starting single phase, asynchronous electrical motors in refrigerator compressors and the like. In particular, the invention relates to solid state thermal relays embodying a wafer of a material with a positive temperature coefficient of electrical resistivity (PTC) of the type well-known to those skilled in the art. The invention includes a process for assemblying the relays.
In motor starting, a resistor element of positive temperature coefficient of resistivity is conventionally arranged in series with the start winding of a refrigerator compressor motor or the like to permit a high initial current in the start winding for starting the motor. The resistor element heats up and stabilizes in a high resistance condition during running of the motor so that the start winding current is reduced to a very low level when the start winding has completed its motor starting function. When motor operation is thereafter interrupted by actuation of a motor protector or otherwise, the resistor element cools to reset for subsequent restarting of the motor. In that arrangement, the operating efficiency of the motor is affected--to some extend at least--by power dissipation in the resistor element during normal running operation of the motor. Further, if motor restarting is attempted before the resistor element has sufficiently cooled, the condition of the resistor element may interfere with proper restarting. It would be desirable to reduce such power dissipation in the resistor element to a very low level and, at the same time, to adapt the resistor to be rapidly cooled after interruption of motor operation to permit prompt restarting of the motor if such restarting should be desired a short time after the interruption in motor operation has occured.
It is an object of this invention to provide a novel and improved thermal relay for starting electrical motors; to provide such a relay which is particularly adapted for starting asynchronous motors for refrigerator compressors; to provide such an improved relay incorporating a resistor element of positive temperature coefficient of resistivity wherein the power dissipation as loss in the resistor element during running operation of the motor is reduced to a very low level and wherein the resistor element is also adapted to cool rapidly after interruption of motor operation for promptly resetting the relay for restarting of the motor; to provide such an improved thermal relay which is adapted for economical assembly; and to provide a novel and improved method for assembling such a relay.